This invention relates to a coupling which transfers a torque from one shaft to another via a keykeyway fit portion having a watertight seal.
As an example of a coupling to transfer torque between two shafts, a cross-type universal coupling shown in FIG. 3 is usually known. In this universal coupling, each of four columns 32 (only one is shown in the figure) of a cross 31 which is a rigid body of cross shape is equipped with needle rollers 33, a thrust washer 34 above such rollers, and an oil seal 35 below such rollers. Each of bearing cases 36 (36r, 36l, 36o, 36u) is rotatably installed on each of the above columns 32 via the needle rollers 33 with a bolt 37. A yoke 38 fixed on an end of a shaft which is not shown in the figure is connected to the bearing cases 36l and 36r on the left and the right by four bolts 39 in such a manner that a keyway 38a and a key 36a, which are formed on their connecting surfaces in the radial direction, will fit together. The upper and lower bearing cases 36o and 36u are similarly connected to another yoke fixed on an end of the other shaft, which is not shown in the figure, with four bolts. Rotation is transferred from one shaft to the other one which crosses the former.
The above bearing cases 36 are each rotatably installed on the columns 32 of the cross via the needle rollers 33 as shown in FIG. 4. The keys 36a are inserted into the respective keyways 38a and the bearing cases 36 are fixed to the yoke 38 by the bolts 39 as shown in FIG. 5. Close fit and transition fit cannot be adopted for the fitting of the key 36a and keyway 38a because of restrictions in the mounting of this universal coupling on the shafts. Therefore, clearance fit is adopted, which results in tiny gaps between torque-transfer contacting surfaces between the key 36a and keyway 38a and between axially opposing surfaces 41 of the same as shown in FIG. 5.
Thus, with the conventional universal coupling mentioned above, the keyways 38a and the keys 36a, which transfer the torque between the yoke 38 which is connected with one of the shafts and the bearing cases 36 which are connected with the other shaft, are clearance-fit as shown in FIG. 6, a detailed view. Therefore, water enters from outside, as shown by the arrows, via the axial gap 42, side grooves 43 and 43 provided on the both sides of the key 36a, and an end 44 of the keyway toward the center of the yoke (See FIG. 3). Such water enters respective tiny gaps between the keys and keyways and will not easily drain. As a result, corrosion and wear are caused by such water in the clearance of the key-keyway fit portions. Consequently, the keys 36a and the keyways 38 will be damaged by corrosion cracking and wear.